DESCRIPTION: Applicant's Abstract The purpose of this study is to compare the effectiveness and cost-effectiveness of two strategies for enrolling and retaining injected drug users (IDUs) in a six-month program to provide them with a full series of three hepatitis B vaccine doses. One method will use periodic contact with street outreach staff, similar to that which has been used successfully in administering directly observed therapy ("DOT") for tuberculosis. The other will use cash incentives such as those that have traditionally been successful in retaining participants in longitudinal research on substance use. Hepatitis B is among the serious clinical medical consequences of drug abuse, and vaccinations administered using similar schedules, such as those for human immunodeficiency virus. Study participants will be recruited from among IDUs participating in the Urban Health Study's ongoing dynamic cohort study in six communities in the San Francisco Bay Area. Cohort participants will be screened with serologic tests for hepatitis B surface antigen (HBsAg), antibody to HBsAg (anti-HBs), and antibody to hepatitis B core antigen (anti-HBc). Enrollment will be offered to those with no serologic markers of hepatitis B and to those with "isolated core antibody" - that is, those with anti-HBc but neither HBsAg nor anti-HBs. Although isolated core antibody is rare in most populations, it is found in 10-35% of IDUs, and its interpretation remains uncertain. IDUs with isolated core antibody will be vaccinated, and their immunologic responses to vaccination will be measured, to answer the question of whether and in what proportion such persons have been or are currently infected with HBV or are susceptible and in need of vaccination. The principle aims of this study are to compare the effectiveness and cost-effectiveness of two methods of delivering a three-dose vaccine series to injecting drug users, and to determine the biological meaning of the isolated core antibody serological profile.